Subsurface removal of arsenic by injection with oxygenated groundwater has been proposed as a viable technology forobtaining ‘safe’ drinking water in Bangladesh. While the oxidation of ferrous iron to solid ferric iron minerals, to whicharsenic adsorbs, is assumed to be driven by abiotic reactions, metal-cycling microorganisms may potentially affect arsenicremoval. A cultivation-independent survey covering 24 drinking water wells in several geographical regions in Bangladeshwas conducted to obtain information on microbial community structure and diversity in general, and on specific functionalgroups capable of the oxidation or reduction of arsenic or iron. Each functional group, targeted by either group-specific 16SrRNA or functional gene amplification, occurred in at least 79% of investigated samples. Putative arsenate reducers andiron-oxidizingGallionellaceaewere present at low diversity, while more variation in potentially arsenite-oxidizingmicroorganisms and iron-reducingDesulfuromonadaleswas revealed within and between samples. Relations betweencommunity composition on the one hand and hydrochemistry on the other hand were in general not evident, apart from animpact of salinity on iron-cycling microorganisms. Our data suggest widespread potential for a positive contribution ofarsenite and iron oxidizers to arsenic removal upon injection with oxygenated water, but also indicate a potential risk forarsenic re-mobilization by anaerobic arsenate and iron reducers once injection is halted.@en